Method and apparatus for comparing electrical transients



A ril 12, I938. 1.. STATHAM 2,113,749 I METHOD AND APPARATUS FOR COMPARING ELECTRICAL TRANSIENTS Filed Dec. 13, 193s s Sheets-Sheet'l .5? 51 .3 I pI I I I P W'l l l lr- A ril 12, 1938. L STATILIAM 2,113,749

METHOD AND APPARATUS FOR COMPARING ELECTRICAL TRANSIENTS Filed Dec. 13, 1935 e Sheets-Sheet 2 E... sTATHAM Aprii 12, was,

Filfid 1390.13, 1935 6 Sheets-Sheet 3 r i I I l L. .l

'4 .2 I I I April 12, 1938, I $TATHAM.I 2,113 '14i9 METHOD AND-APPARATUS FOR COMPARING ELECTRT'CALTHANSIENIS Filed Dec. 13, 1935 6 Sheets-Sheet 4 April 12; 19330 s 'l-m m 2,113,749 METHOD AND APPARATUS FOR COMPARING ELECTRICAL TRKNSIENI'S 7 Filed Deq. 1a,, 1935 s sheets sheet s Apfil 12,1938. 1 HA 2,113,749

METHOD AND APPARATUS FOR COMPARING ELECTRICAL TBANSIENTS Filed Dec. 15, 1935 s sheets-sheet s 2552i. worm 175522.. mom-n; 5

Patented a i, was

that

Louis Statham, Houston,

Tex, assignor to Standard Oil Development Company, a corporation of Delaware Application December 13, 1935, Seriai No. t,1"19

20 Claims.

This invention relates to the comparison of electrical transients.

More particularly it relates to improvements in methods and apparatus for geophysi cal prospecting by means of earth 5 electrical transients.

In geophysical prospecting by means of earth electrical currents as previously carried out two methods of one method through the made of the both by indu other method the instantaneous potential at difrent points due to a suddenly applied current has 1 been recorded and the time constants of the transients studied to obtain determinations of the conductivity fundamental of the earth. The latter method is 1y superior to the method involving measurements of the steady potentials because a greater depth can be reached by the same electrode spacing. This latter method has beendescribed in U. 23, 1933, to

S. Patent No. 1,911,137 granted May Ludwig W. Blau, entitled Method and apparatus for geophysical exploration. The field procedure of the method of this patent has been described in detail by J. C. Karcher and Eugene McDermott in the can Association of Petroleum 19 No. 1 (January 1935) page method involves the recording,

Bulletin of the Ameri- Geologists, voliune 64. The use of the with time marks,

of the transient.

The transient obtained by this method is then compared with other transients taken in a similar manner at other stations.

The usual method of evaluating the time of the transient has been to measure the time required for the potential to reach a. predetermined fraction of its steady value. The

time thus measured ,has then been used to calculate the conductivity of the earth at that station, this time being a conductivity rent electrod fined methods of timing function of the and of the distance between the cures and the potential electrodes. Re-

have been required, as

the time involved is very short (usually less than 1/100th of a second). It has been necessary also to record the transient with very great amplitude in order that small variations could be detected.

If a variatio appear on the record n of 1/10th of as a one percent were to displacement of one millimeter in the position of the trace of the transient, the amplitude of the recorded transient must be 100 centimeters. with such an amplitude tain.

It is an obj A record of this nature has been difiicult to .ob-

ect of this inventionto eliminate the (Cl. Ilia- 182) necessity of making time measurements and of recording such large amplitude transients, and to accomplish at the same time an accurate comparison of the transient at one station with the transient at another station.

Other objects will be apparent from the specification, taken in connection with the accompanying drawings in which latter Fig. l is a transverse sectional view through the earth showing in diagrammatic representation a 10 preferred form of apparatus for carrying out the invention.

Fig. 2 is a top plan view showing the arrangement of electrodes in Fig. 1.

Fig. 3 is a top plan view showing a modified 15 form of arrangement of the electrodes.

' Fig. 4 is a top plan view showing still another modified arrangement of the electrodes.

Fig. 5 is a vertical sectional view through the earth showing an arrangement for tying together the results obtained over an area.

Fig. 6 is a top plan view showing the arrangement of electrodes of Fig. 5 disposed along a roadway.

Fig. '7 is a record of the potential difference between the potential electrodes due to the current between the first set of current electrodes.

Fig. 8- is a record representing the potential difference between the potential electrodes due to the second set of electrodes.

Fig. 9 is a record of the sum of these two potential differences.

Fig. 10 is a vertical earth showing a coil by induction.

Fig. 11 is a record showing the potential induced in the coil due-to the current between the first set of electrodes. I I

Fig. 12 is a record showing the potential induced in the coil due to the current in the second set of electrodes.

Fig. 13 is a record showing the sum of the two induced potentials, and

Fig. 14 is a vertical sectional view through the earth showing diagrammatically an arrangement of stations used which resulted in locating a fault.

Referring particularly to Figs. 1 and 2 of the drawings, reference numerals I and 2 designate a pair of current stakes or electrodes driven into the ground at suitable positions. A second pair of current electrodes 4 and 5 are disposed in the ground in alignment longitudinally with the first pair of electrodes. The two pairs of electrodes are spaced from each other any suitable distance such as 2000 feet. The distance between the elecsectional view through the for receiving the transient trodes I and 2 may be any suitable distance such as one mile. Preferably the distance between the electrodes 4 and 5 is made the same as the dis- 5, the means comprising thyratron tubes I0 and.

II, which may be of the screen grid type; the plates 36 and 31 of the tubes are connected through lines 26 and 21 to current stakes 2 and 4. The grids I2 and I3 of the tubes are connected togetherthrough lines I4 and I5 and resistances I6 and IT. The resistances I5 and I1 are connected through resistance I8, battery 20 and resistances 2| and 22 to the cathodes 23 and 24 of the thyratron tubes. The resistance I8 and battery 20 are shunted by a firing switch 25. The cathodes of the tubes are also connected through rheostats 30 and 3| and batteries 32 and 33 through lines 34 and 35 to the current stakes and 5. The battery 20 is of such value that no current is permitted to flow through the thyratron tubes from plate to cathode so long as the firing switch 25 is open. Upon closing the firing switch 25 current is caused to flow from plate to cathode of the two thyratrons I0 and I I simultaneously. While the arrangement of thyratron tubes has been illustrated as a preferred means for closing the circuits between the two sets of electrodes simultaneously it will be understood that other mechanisms for this purpose such as mechanical means can be used and some of the advantages of the invention will be, retained.

A few lines of current flow between electrodes and 2 are designated by 40 and lines of current flow between lines 4 and 5 are designated by 4|. The directions of flow of currents are shown by arrows 42 and 43 and are preferably in opposite directions as shown.

Upon closing the firing switch 25 the lines. of current flow 40 and 4| are propagated downward and outward beginning with a concentration of current near the surface and progressing with a broad distribution of the current linesthrough a large volume. This propagation of current lines gives rise to a transient potential at all points in said volume of the earth.

In a preferred form of the invention the thyratron tubes IO and II are of the screen grid type in order that the resistances l6, l1, I8, 2|, and 22 which couple the two independent current circuits may be large, thereby eliminating the possibility of interchange of energy between the two current circuits, as it is not desired that current flow between electrodes 2 and 5 or between elec-,- trodes I and 4. By way of example, the resistances I6, |'I'., I8, 2|, and 22 maybe 100,000 ohms each and the battery .20 may be 22 volts. The

rheostats 30 and 3| may be ohms each and the batteries 32 and 33 may be of 100 volts each.

The electrodes I, 2,4. and 5 may consist of a short section of copper pipe immersed in a shallow hole. containing salt water. The potential electrodes 44 and 45 may be ordinary brass rods driven into the ground to a depth of about one foot.

Means are provided for measuring the difference between the transient potentials at two selected points in the earth. The means comprises spaced potential electrodes 44 and 45 which are disposedbetween the two setsv of current electrodes and in substantial alignment therewith. The potential electrodes 44 and 45 are connected by lines 46 and 41 to a suitable amplifier 48 and an oscillograph 49. Preferably the oscillograph is a cathode ray oscillograph.

When the switch 25 is closed, currents 40 and 4| are caused to flow simultaneously through the earth producing at the potential stakes 44 and 45 a diiference of potentialwhich is equal to the sum of the potential differences induced by current 40 and that induced by current 4|. The steady state potential difierence induced between 44 and 45 by current 40 is made equal and opposite to that induced by current 4| by adjusting rheostats 30 and 3 I. When the steady state values of the potential difierences are equal and opposite,

anything which is recorded must be the difierence between the transients. Preferably the record is produced on a cathode ray oscillograph screen.

In a preferred form of the invention the cathode ray oscillograph which is used is of the 4 plate electro-static deflection type whose horizontal deflection plates are connected to a linear sweep circuit and whose vertical deflection plates are connected to the voltage to be recorded and which can be used to impress upon the fluorescent screen any transient phenomena which it may be desired to study by visual observation without the necessity of employing photography.

The potentials involved are illustrated in Figs. 7, 8, and 9. In Fig. '7 the curved line 41 leading from a base line 48 represents the potential difference between 44 and 45 induced by the current 40 in the absence of the current 4|. In Fig. 8 the curved line 49 represents the potential difference between 44 and '45 induced by current 4| in the absence of current 40. The two potential diilerences are equal in amplitude and opposite in phase as clearly shown by Figs. '1 and 8 due to the opposition of the currents 40 and 4| as shown by the current direction arrows 42 and 43. The potential difference represented by Fig. 8 is assumed to rise more rapidly than that repre sented by Fig. 7. The sum of the two potential differences is shown in Fig. 9 in which the curved line 50 represents the potential difference between potential electrodes 44 and 45 induced by the application of both currents 40 and 4| simultaneously, and, referring to Figs. 7 and 8, it is the raphical sum of these two transients. The potential as'shown in Fig- 9 has a negative value at A due to the fact that the rate of increase of potential difference as shown in Fig. 8 is greater than that shown in Fig. '7 and the potential difference as shown in Fig. 9 meets its equilibrium position at B due to the fact that ultimate amplitudes of the potential differences as shown in Figs. 7 and 8 are equal and opposite. We then move 44 or 45 or both closer to electrode 2 thereby increasing the rate of rise of potential difi'erence due to current 40 or decreasing the rate of rise of potential difference due to current4'l; The steady state amplitudes of the potential differences induced by currents 40 and 4| are then again made equal and opposite by adjusting rheostats 30 and 3| and another observation is made. When 44 and 45 have finally been adjusted so that the transients are equal and opposite the record on the oscillograph screen will appear as a straight line, and knowing that the time conarran e at by earth current 00, nor is the potential at 60 influenced appreciably by the earth current M. The potential difference between 30 and 45 as recorded by the recording apparatus, will then be zero when the potential at M is equal to the potential at 5. The setup 5, Qand it may then be far removed from the setup 5, d and 05 and placed-by each station represents the direction and distance which the potential electrodes had to be moved in order to obtain cancellation of the transients and this arrow points in the direction of greater effective conductivity. Referring to station Bi, it was found that one of the potential electrodes had to be moved 75 feet in the direction indicated in order to obtain completecancellation of the transients, showing that the conductivity of the earth was increasing toward the right as viewed in Fig. 14. Station 02 was set up approximately 7000 feet from station M. The same observation was taken at station 02 and it was found to be necessary to move a potential electrode 25 feet toward the left as viewed in Fig. 14 to obtain complete cancellation of the transients thereby indicating that the conductivity of the earth was increasing slightly toward the left. Another station was taken approximately 7000 feet from station 62 at 6 1 at which-it was foundto be necessary to move a potential electrode 600 feet toward the right as viewed in Fig. 14 thereby indicating that the conductivity of the earth was increasing very rapidly toward the right at that point. Another station 06 was taken approximately 7000 feet from station 60 at which it was found to be necessary to move a potential stake 175 feet toward the right as viewed in Fig. 14 in order to obtain a complete cancellation of the transients thereby indicating that the conductivity of the earth was continuing to increase toward the right at that point. Another station 0'! was taken approximately 6000 feet from station 66 at which it was found necessary to move a potential stake 100 feet toward the right as viewed in Fig. 14 thereby indicating that the conductivity of the earth was still increasing toward the right at that point. Another station 68 was taken approximately 7000 feet from station bl atwhich it was found necessary to move a potential stake 25 feet toward the right as viewed in Fig. 14 thereby indicating that the conductivity of the earth was increasing very slightly toward the right at that point. Upon observation of the results obtained at each of these stations, it was noted that the amount of change of conductivity toward the right had been. a, maximum at station ,60 and had decreased gradually at stations 06, 61 and 68 thereby suggesting the possibility-of existence ofa fault near station 64 and a gradual up-lift extending as far as station 01. Station 53' was then taken approximately 1000 feet to the left of station M as viewed in Fig. 14 at whichit was found necessary to move apotential stake 250 feet toward the right in order to obtain'complete cancellation ofthe transients, thereby indicating that the conductivity of the earth was increasing toward the right. Station 00 was" taken approximately 1000 feet to the right of station 60 as viewed in Fig. 14 and it was found necessary to move a potential stake 300 feet toward the right in order to obtain complete cancellation of the transient, thereby indicating that the conductivity of the earth was still increasing greatly toward the right. The largest anomaly then seemed to be localized in the neighborhood of station t t and in order to confirm our interpretation, shallow core wells were drilled at several-points between stations 02 and 60 disclosing the presence of a fault lying approximately below station 6 and dipping to the right as viewed in Fig. 14 as shown at E0. These core wells were drilled to a depth of 250 feet at which depth the fault was in evidence with a throw of 20 feet.

Referring to Fig. 3, a modified arrangement 7 of the electrode is illustrated in top plan view with current electrodes 80 and M on a straight line with their corresponding potential electrode 82, and current electrodes 83 and 8d lying in approximately the same straight line with their corresponding potential electrode 85. It is noted that it is not necessary-to have the two halves of the setup lying in the same straight. line.

Referring to Fig. 4 a still further modified arrangement is shown in top plan view comprising current electrodes 06 and 8,? with their corresponding potcntial'electrode 80, and current electrodes 00 and 00 with their corresponding potential electrode 0!. The two halves of the. setup are located in widely different neighborhoods. The apparatus illustrated in Figs. 3 and 4 is used in exactly the same manner as the apparatus in not be straight, and like reference numerals have been applied to like parts as in Figs. 1 and 2.

Instead of the arrangement illustrated in Fig. 1 for receiving the transient by conduction, an arrangement is illustrated in Fig.- 10 for receiving the transient by induction. The apparatus in Fig. 10 comprises an inductance coil 500 which may be a loop or solenoid whose axis lies parallel to the surface of the ground lilland perpendicular to the line between current electrodes Hi2 and W3. If it is necessary to dispose current electrodes 403 and W0, and H02 and M5 in a configuration such as is illustrated in Fig. 4, then two inductance coils may be employed, using one for each setup, of current electrodes and con-' necting the two inductance coils in series, the axis of each inductance coil lying parallel to the ground and perpendicular to the line drawn through the corresponding current electrodes.

The transients now involved are illustrated in Figs. 11, 12, and Y13. Referring to Fig. 11, the curve designated M2 represents with respect to a base line lit the transient induced in the inductance coil E00 by the current I06. 12, curve M l represents with respect to the base line the transient induced in the inductance coil E00 by the current till. Fig. 13 represents by curve H 5 the sum of the two transients. The two transients are made to be initially equal in amplitude and opposite in phase, as clearly shown In Fig.

in Figs. 11 and 12, due to the opposition of the sum of the two transients as-shown in Fig. 13 represents the transient induced into the inmade.

rents I06 and I01 simultaneously. Referring to Figs. 11 and 12 it is the graphical sum of these transients. The transient as shown in Fig. 13 lies above the line H3 at A due to the fact that the rate of decrease of the transient as shown in Fig. 12 is greater than that of the transient as shown in Fig. 11. We then move the induc tance coil I closer to I02 thereby increasing the rate of decrease of the transient as represented by Fig. 11 and decreasing the rate of decrease of the transient as shown in Fig. 12. The initial amplitudes of the two transients are then again adjusted to be equal and opposite by suitable rheostats and another observation is When the position of the inductance coil |00has been so adjusted that the transients as represented in Figs. 11 and 12 are equal and opposite, the record on the oscillograph screen will appear as a straight line. The distances from I00 to I02 and I00 to I03 should then be carefully measured, and from these data the relative effective conductivities of the earth below each half of the setup may be computed as before.

Referring to Fig. 5, an example of field procedure is shown in which i, 2, 4, and are current electrodes and 44 and 45 are potential electrodes; the distances between electrodes may be as previously described in Fig. 1. After a reading has been made with this setup the current electrodes l and 2 may be moved to I and 2 and the potential electrodes 44 and 45 moved to 44 and 45 leaving the current electrodes 4 and 5 in their initial position. The reading which may now be taken furnishes a comparison of the electrical properties of the earth beneath electrodes I and 2 with those beneath electrodes I and 2' and by continuing this procedure continuous comparisons can be made over long distances.

By the invention described an accurate comparison of the transient at one station with the transient at another station is obtained without the necessity of making time measurements. Furthermore, it is not necessary to record photographically at all. If a cathode ray oscillograph with a phosphorescent screen is used, the traces are visible on the screen for a sufliciently long time for the operator to ascertain which way one of the stakes 44 and 45 must be moved to obtain a balance. Ground can be covered rapidly; thus inactual practice the distance from electrode l to 2 and from 4 to 5 may be one mile, distances 2 to 44 and 45 to 4 may be about 500 feet and the distance from 44 to 45 may be about 1000 feet. Then, as illustrated in Fig. 5, current stakes I and 2 are moved to new positions I and 2 and potential stakes 44 and 45 are moved to 44 and 45'. It is evident, then, that in the first setup the efiective conductivity of the ground below electrodes ,l and 2 is compared with the efifective conductivity of the ground below electrodes 4 and 5. In the second setup the efiective conductivity of the ground below electrodes 4 and 5 is compared with the efiective conductivity of the ground below electrodes I and 2'; thus one obtains a comparison of the effective conductivities from one setup to the next and covers upward of one mile per setup with the distances commonly employed.

While the invention has been described as applied to the surface of the earth it will be un derstood that it can be carried out in water, under water, along the'sides of cliffs or other vertical structures, in bore holes, or in mines.

ductance coil I00 by the application of both cur- Also, it-can be applied to the determination of the electrical properties of other materials such as metals, semi-conductors and insulators.

Various changes and alternative arrangements may be made within the scope of the appended claims, in which it is my intention to claim the a logical structures, which comprises passing an electric current through the earth between at least two pairs of spaced electrodes, simultaneously altering the electric current through the ground between each pair of electrodes whereby a transient is obtained for each set of electrodes, simultaneously receiving the transients, and varying the position of the receiving means until the substantial cancellation of the transients is obtained. p.

3. The method for the determination of geological structures, which comprises passing an electric current through the earth between at least two pairs of spaced electrodes, simultaneously altering the electric current through the earth between each pair of electrodes whereby a transient is obtained for each set of electrodes, receiving the transients, and varying the relative positions of the receiving means and the electrodes whereby substantial cancellation of the transients is obtained.

4. The method for the determination of geological structures, which comprises disposing at least two pairs of spaced electrodes in the earth, simultaneously applying an electric current through the earth between each pair of electrodes whereby a transient is obtained for each pair of electrodes, receiving the transients, and adjusting the amplitude of the current for each pair of electrodes independently in order that the effect of the direct current components of the transients can be cancelled at the receiving means.

5. Apparatus for the determination of geological structures, which comprises at least two pairs of spaced electrodes in the'earth, means for simultaneously applying an electric current through the earth between each pair of electrodes whereby a transient is obtained for each pair of electrodes, receiving means including an electric circuit connected to a pair of potential electrodes disposed in the earth between the pairs of electrodes for observing a predetermined combination of the transients.

6. Apparatus for v the determination of geological structures, which comprises at least two pairs of spaced electrodes in the earth, means for simultaneously applying an electric current through the earth between each pair of electrodes whereby a transient is obtained for each pair of electrodes, receiving means including an electric circuit connected to an inductance coil disposed on the surface of the earth in spaced relation to the pairs of electrodes for observing a resultant of the several transients.

'7. Apparatus for the determination of geological structures, which comprises at least two tween each pair of at least arrayed cancellation of the efiect of the transients upon the receiving means is obtained.

8. The method for the determination of electrical properties of matter which comprises initiating a difference in potential between each pairof at least two pairs of spaced electrodes in the matter whereby a transient is obtained for each pair of electrodes and observing a resultant of the several transients.

9..I'he method for the determination of subsurface earth formations which comprises simultaneously initiating a difference in potential betwo pairs of spaced electrodes in the earth whereby a transient is set up in the earth between each pair of electrodes and observing a resultant of the several transients.

10. The method for the determination of earth substructures which comprises arranging at least two pairs of spaced electrodes in the earth, each pair of electrodes being part of a circuit including a source of electrical power and an adjustable resistance, impressing the same voltage across the several pairs, of electrodes, adjusting the resistances in the several electrode circuits until a current of the same amplitude passes through the earth between each pair of electrodes, simultaneously initiating the same difierence in potential between each of the several pairs of electrodes whereby a transient is set up in the 1 earth between each pair of electrodes and observing a resultant of the several transients. 11. The methodfor the determination of earth substructures which comprises simultaneously initiating a difference in potential between each pair of at least two pairs of spaced electrodes in the earth whereby a transient is set up in the earth between each set of electrodes and substantially cancelling the transients against each other.

12. The method for the determination of earth substructures which comprises simultaneously initiating the same difference in potential between each pair of at least two electrodes in the earth whereby a transient is set up in the earth between each pair of electrodes,

simultaneously receiving the several transients, and varying the position of the receiving means until the substantial cancellation of the transients is obtained.

13. The method for determination of earth substructures which comprises arranging at least two pairs of spaced electrodes in alignment on the earth's surface, initiating a difference in potential between the electrodes of each pair whereby a transient is set up in the earth between each pair of electrodes and observing a resultant oftheseveral transients.

14. The method for the determination of earth substructures which comprises arranging at least two pairs of electrodes and in spaced relation on the earths surface, ar-

ranging a receiver of earth currents between adjacent pairs of electrodes and in alignment therewith, simultaneously initiating the same dif ference in. potential between the electrodes of each pair, said difference in potential being in op-.

disposed in the earth,

'multaneously applying an pairs of spaced in longitudinal alignment I posite directions in adjacent pairs of electrodes whereby a transient is set up in the earth between the electrodes of each pair and the transients of adjacent pairs are in opposite di' rections, receiving the transients of adjacent pairs of electrodes in said receiving means and observing a resultant of said transients.

15. The methodfor the determination of earth substructures which comprises arranging at least two pairs of electrodes. in longitudinal alignment and in spaced relation on the earths surface,

each pair of electrodes being part of a circuit including a source of electrical power and an adjustable resistance, arranging a receiver of earth currents between adjacent pairs of electrodes and in alignment therewith, impressing the same voltage across the several pairs of electrodes, adjusting the resistances in the several electrode circuits until a current of the same amplitude passes through the earth between each pair of electrodes, simultaneously initiating the same difierence in potential between the electrodes of each pair, said difierence in potential being in opposite directions in adjacent pairs of electrodes whereby a transient is set up in the earth'between ,the electrodes of each pair and the transients of ad.- jacent pairs are in opposite directions, receiving the transients of adjacent pairs of electrodes in said receiving means and observing a resultant of said transients.

16. Apparatus for the determination of geological structures comprising at least two pairs of spaced electrodes in the earth, means for simulis set up in the earth between the electrodes of each pairand means capable of recording a resultant of the several transients arranged in spaced relation to the pairs of electrodes.

17. Apparatus for the determination of earth substructures which comprises at least two pairs of spaced electrodes in'the earth, means for sielectric current through the earth between each pair of electrodes whereby a transient is obtained for each pair of electrodes and receiving means including an electric circuit connected to a pair of potential electrodes disposed in the earth in spaced relation to the pairs of electrodes for observing a resultant of the several transients.

18. Apparatus for the determination of earth substructures comprising at least two pairs of spaced electrodes in the earth, each pair of electrodes being a part of an independent electrical circuit including a source of electrical power, a variable resistance in each of said circuits whereby with the same'voltage on each pair of electrodes the earth currents between the several pairs of electrodes can be adjusted to the same amplitude, means for simultaneously initiating the same difference in potential between each pair of electrodes whereby a transient is set up in the earth between each pair of electrodes and means arranged in spaced relation to the several pairs of electrodes for observing a resultant of the several transients.

I9. Apparatus-for the determination of earth substructures comprising a plurality oflongitudinally aligned pairs of electrodes in spaced relation to each other, means for simultaneously initiating a difference in potential between each pair of electrodes whereby a transient is set up in the earth between each pain-of electrodes and means disposed between adjacent pairs of electrodes and in longitudinal alignment therewith trodes and the plate and filament of the other vacuum tube being connected in the circuit of the other electrode and the grids of the two tubes being connected to each other through a resistance, means connecting the resistance to each of said circuits through a battery and a by-pass line including a switch connecting said resistance to said circuits in such a manner that when the.

switch is closed the battery is cut out of the circuit.

LOUIS STA'I'HAM. 

